Similarly, in the case of feeble velocities, in the new mechanics as well as the old, bodies perceptibly oppose the same resistance of inertia to the forces which tend to accelerate their movement as to those which tend to alter the direction, to give a curve to their trajectories. In the case of great velocities that is not so. Mass, therefore, increases rapidly with velocity. It becomes infinite when the velocity equals that of light. No body whatever can attain or surpass the velocity of light, because, in order to pass that limit, it would need to overcome an infinite resistance. In order to make it quite clear, let us give certain figures which show how mass varies with velocity. The calculation is easy, thanks to the formula which we have previously seen, giving the values of the Fitzgerald-Lorentz construction. A mass of 1,000 grammes will weigh an additional two grammes at the velocity of 1,000 kilometres a second. It will weigh 1,060 grammes at the velocity of 100,000 kilometres a second; 1,341 grammes at the velocity of 200,000 kilometres a second; 2,000 grammes (or double) at the velocity of 259,806 kilometres a second; 3,905 grammes at the velocity of 290,000 kilometres a second. * * * * * That is what the new theory tells us. But how can we verify it? It would have been impossible only fifty years ago, when the only velocities known were those of our vehicles and projectiles, which then did not rise, even in the case of shells, above one kilometre a second. The planets themselves are far too slow for the purpose of verification. Mercury, for instance, the swiftest of them, travels at a speed of only a hundred kilometres a second, which is not enough. If we had at our disposal no higher velocities than these, we should have no means of settling which was right, the classical mechanics with its constancy of mass or the new mechanics with its assertion of variability. It is the cathode rays and the Beta rays of radium which have provided us with velocities great enough for the purpose of verification. These rays consist of an uninterrupted bombardment by small and very rapid projectiles, each of a mass less than the two-thousandth part that of an atom of hydrogen, and charged with negative electricity. They are the _electrons_. The cathode tubes of radium give out a continuous bombardment of these minute projectiles, charged, not with melinite, but electricity: far smaller than the shells of our artillery, but animated with infinitely greater initial speeds. The velocity of “Bertha’s” shells is contemptible in comparison. But how was it possible to measure the speed of these projectiles?